Chiral Separation and Enantiomer Resolution (5C) - Chem & Physics

1. A racemic chiral thiol is resolved by chiral HPLC and collected into two fractions. When each fraction is reinjected separately under identical conditions, each produces a single peak at the same retention time as originally observed for that fraction. Based on the technique used, which conclusion is most consistent with the results?
   Answer: Each collected fraction is stable and compositionally consistent (no significant racemization or cross-contamination) on the HPLC timescale. The skill being tested is verifying fraction stability and purity after chiral HPLC resolution through reinjection. Reinjection assesses if fractions maintain composition without racemization or contamination on the chromatographic timescale. Here, each fraction reinjects as a single peak at original retention times, confirming stability. Choice A accurately concludes the fractions are stable and consistent. A common distractor like choice B fails by misinterpreting as diastereomers, ignoring enantiomer stability. A transferable insight is to use reinjection as a purity check in resolution workflows. In similar experiments, monitoring over time detects potential racemization.
2. A racemic chiral acid is resolved by forming diastereomeric esters with an enantiopure alcohol and separating them by achiral GC. The chemist claims the GC separation directly proves enantiomeric separation of the original acid without further steps. Based on the technique used, which conclusion is most consistent with the process?
   Answer: GC separated diastereomeric derivatives; additional steps are required to regenerate the acid enantiomers and confirm their ee. The skill being tested is distinguishing diastereomer separation from direct enantiomer resolution in GC. Derivatization creates diastereomers separable on achiral columns, but regeneration and ee confirmation are needed for the original compound. In this case, diastereomeric esters are separated, but claiming direct proof for the acid overlooks these steps. Choice A accurately states that GC separates derivatives, requiring additional steps for confirmation. A common distractor like choice B fails by suggesting achiral GC directly resolves enantiomers, which it cannot. A transferable check is to hydrolyze derivatives and analyze by chiral methods. In similar indirect resolutions, selecting volatile derivatives enhances GC suitability.
3. A racemic chiral alcohol is analyzed by gas chromatography (GC) using an achiral capillary column at 1 atm with a temperature program that fully elutes the compound. A single GC peak is observed. The same sample analyzed by chiral HPLC shows two peaks. Based on the technique used, which conclusion is most consistent with the results?
   Answer: The GC result is consistent with enantiomers having identical physical properties in an achiral environment, while chiral HPLC can resolve them via differential interactions. This question tests understanding of enantiomer behavior in achiral versus chiral chromatographic systems. Enantiomers have identical physical properties in achiral environments, including identical boiling points, vapor pressures, and retention times on achiral GC columns, which explains why only a single peak appears in the achiral GC analysis. However, when analyzed by chiral HPLC with a chiral stationary phase, the enantiomers form diastereomeric interactions with the chiral selector, resulting in different retention times and two peaks. The correct answer (A) accurately explains this fundamental difference between achiral and chiral separation environments. Choice B incorrectly concludes the compound is achiral based on the GC result, failing to recognize that achiral methods cannot distinguish enantiomers. A critical insight is that observing a single peak in achiral chromatography never rules out the presence of enantiomers - chiral analysis is required to detect and separate them.
4. A racemic drug is resolved by chiral HPLC to give two fractions. The first fraction has 70% of the total mass collected, the second 30%. The original injection was a true racemate and detector response is linear and identical for both enantiomers. Which statement best describes the outcome of the separation process?
   Answer: The unequal collected masses most likely reflect unequal recovery/collection efficiency rather than an enantiomeric imbalance in the injected racemate. The skill being tested is interpreting mass recovery in chiral HPLC fractions from racemic mixtures. Unequal masses can arise from collection inefficiencies despite equal chromatographic areas in true racemates. In this vignette, the 70:30 mass split occurs despite racemic input and linear detection, pointing to recovery differences. Choice A correctly attributes the imbalance to collection efficiency. A common distractor like choice B fails by assuming HPLC preserves ratios perfectly, overlooking practical losses. A transferable check is to integrate peak areas before and after collection for mass balance. In similar preparative separations, automating collection minimizes variability.
5. A pharmaceutical lab is resolving the enantiomers of a chiral beta-blocker candidate (Drug X) because only the (S)-enantiomer shows high affinity for a cardiac receptor, while the (R)-enantiomer produces dose-limiting dizziness. A racemic mixture (1.00 mg/mL in 50:50 acetonitrile:water) is analyzed by HPLC at 25C using a chiral stationary phase (polysaccharide-derived) and UV detection at 254 nm. The mobile phase and flow rate are held constant. Two well-resolved peaks are observed with identical UV response factors for both enantiomers. Peak areas are 62 and 38 (arbitrary units). The enantiomer eluting first is independently identified (by polarimetry of the collected fraction) as (S)-Drug X. Based on the technique used, which conclusion is most consistent with the results?
   Answer: The sample contains 62% (S)-Drug X and 38% (R)-Drug X by mass, and the earlier elution of (S) indicates weaker interaction with the chiral stationary phase. This question tests understanding of chiral HPLC separation and the interpretation of peak areas for enantiomeric composition determination. In chiral HPLC, enantiomers interact differently with the chiral stationary phase due to diastereomeric interactions, resulting in different retention times. The (S)-enantiomer elutes first, indicating it has weaker interactions with the polysaccharide-based chiral stationary phase compared to the (R)-enantiomer. Since the UV response factors are identical for both enantiomers, the peak areas directly reflect the mass composition: 62% (S)-Drug X and 38% (R)-Drug X. Choice C incorrectly claims enantiomers must have different UV extinction coefficients, but the question explicitly states they have identical response factors. A key insight is that earlier elution in chiral chromatography indicates weaker stationary phase interactions, not stronger ones as suggested in choice B.
6. A racemic drug is resolved by chiral HPLC into enantiomers E1 and E2. Only E1 is intended for clinical use. During stability testing at 37°C in aqueous buffer (pH 7.4), the isolated E1 sample shows a gradual decrease in optical rotation magnitude over 7 days, approaching zero, with no change in LC-MS mass. Based on these observations, which conclusion is most consistent with the results?
   Answer: E1 is undergoing racemization in buffer, reducing enantiomeric excess over time without changing molecular mass. This question tests understanding of chiral separation and the phenomenon of racemization, where an enantiomerically pure compound converts to a racemic mixture over time. Racemization occurs when a chiral center undergoes inversion, typically through deprotonation-reprotonation mechanisms or other pathways that temporarily remove the stereocenter, allowing both enantiomers to form with equal probability. In this scenario, the isolated E1 enantiomer shows decreasing optical rotation magnitude approaching zero, which is the hallmark of racemization - as E1 converts to a 50:50 mixture of E1 and E2, the opposite rotations cancel out. The correct answer (A) accurately identifies this process, noting that racemization preserves molecular mass since E1 and E2 are isomers with identical molecular formulas. Answer B incorrectly suggests decomposition, but this would change the molecular mass detected by LC-MS; answer C misinterprets the physics of optical rotation, as concentration changes would affect the magnitude but not drive it specifically toward zero; answer D fundamentally misunderstands that the original chiral HPLC separation confirmed the compound's chirality. A key insight for similar problems is that optical rotation approaching zero specifically indicates racemization rather than degradation, and this process maintains molecular integrity while losing stereochemical purity.
7. A racemic carboxylic acid drug is resolved by chiral HPLC. The analyst changes only the column temperature from 25C to 45C while keeping flow rate and mobile phase constant. At 25C, the enantiomer peaks are baseline-resolved; at 45C, the peaks partially overlap. Which factor would most influence the separation efficiency in this scenario?
   Answer: Increased temperature can reduce differential binding to the chiral stationary phase, decreasing selectivity. The skill being tested is understanding factors affecting chiral separation efficiency in HPLC, particularly temperature's role in enantiomer resolution. Temperature influences selectivity by altering the thermodynamics of enantiomer-stationary phase interactions, often reducing differences at higher temperatures. In this scenario, raising the temperature from 25°C to 45°C causes resolved peaks to overlap, demonstrating decreased chiral recognition. Choice A correctly identifies that increased temperature reduces differential binding, decreasing selectivity and thus resolution. A common distractor like choice B fails by claiming temperature converts enantiomers to diastereomers, misconstruing that no covalent change occurs. A transferable check is to plot ln(α) versus 1/T (van't Hoff plot) to predict temperature effects on selectivity. In similar optimizations, balancing temperature with flow and mobile phase composition enhances overall separation performance.
8. A research group uses chiral HPLC to assess the enantiomeric purity of a chiral analgesic intermediate (Compound Y). At 30C and constant mobile phase composition, the chromatogram shows two partially overlapping peaks with retention times $t_{R1}=5.10\,\text{min}$ and $t_{R2}=5.60\,\text{min}$. The column dead time is $t_M=1.00\,\text{min}$. Baseline peak widths are $w_1=0.40\,\text{min}$ and $w_2=0.40\,\text{min}$. The team considers changing only one parameter: switching to a longer chiral column of the same stationary phase chemistry and particle size, while keeping flow rate constant. Which factor would most influence the separation efficiency (resolution) under this single change?
   Answer: Increased number of theoretical plates due to greater column length, improving resolution if selectivity is unchanged. This question tests understanding of how column length affects resolution in chiral HPLC separations. Resolution depends on three factors: selectivity (α), retention (k'), and efficiency (N, number of theoretical plates). Increasing column length while maintaining the same stationary phase chemistry and particle size directly increases the number of theoretical plates (N), which improves resolution according to the resolution equation Rs = (√N/4) × (α-1)/α × k'/(1+k'). The longer column provides more opportunities for differential interaction between enantiomers and the chiral stationary phase, leading to better separation. Choice B incorrectly suggests UV absorbance changes with column length, but detector response is independent of chromatographic separation. Choice C contradicts the fundamental principle that longer columns enhance, not eliminate, chiral recognition. When optimizing chiral separations, increasing column length is a straightforward way to improve resolution without changing selectivity.
9. A medicinal chemistry team attempts to resolve a racemic amine (Compound Z) by forming diastereomeric salts with an enantiopure acid (L-tartaric acid). Equal moles of racemic Z and L-tartaric acid are dissolved in ethanol and allowed to crystallize at 20C. The first crop of crystals is collected and converted back to free base Z. Polarimetry of the recovered Z (path length 1.00 dm) gives an observed rotation $\alpha_{obs}=+6.0^\circ$ at a concentration of $c=1.0\,\text{g}/100\,\text{mL}$. The literature specific rotation of enantiopure (+)-Z under these conditions is $[\alpha]=+12.0^\circ$. Based on the technique used, which conclusion is most consistent with the results?
   Answer: The recovered Z is enriched to 50% enantiomeric excess in (+)-Z, consistent with partial resolution via diastereomeric salt crystallization. This question tests understanding of classical resolution via diastereomeric salt crystallization and the calculation of enantiomeric excess from optical rotation data. When racemic amine Z forms salts with enantiopure L-tartaric acid, two diastereomeric salts are created with different solubilities. The observed specific rotation [α]obs = +6.0° compared to the enantiopure value [α]pure = +12.0° indicates partial resolution. The enantiomeric excess is calculated as ee = ([α]obs/[α]pure) × 100% = (6.0/12.0) × 100% = 50%. This means the recovered Z contains 75% (+)-Z and 25% (-)-Z, giving a 50% enantiomeric excess in (+)-Z. Choice A incorrectly claims crystallization cannot separate enantiomers, ignoring that diastereomeric salts have different physical properties. Choice C wrongly assumes any optical rotation indicates enantiopurity. The key principle is that diastereomeric salt formation converts the enantiomer separation problem into a diastereomer separation problem, which can be achieved by crystallization.
10. A racemic alcohol is analyzed on a chiral GC column at 120°C. Two peaks are observed with equal integrated areas and baseline separation. A second run is performed with the same column but the injector temperature is increased from 150°C to 250°C; peak areas remain equal, but the separation decreases (peaks partially overlap). Which factor would most influence the separation efficiency in this scenario?
    Answer: Increasing injector temperature reduces analyte–stationary phase interaction time, decreasing chiral selectivity and resolution. This question tests understanding of temperature effects on chiral GC separation efficiency. In gas chromatography, the injector temperature affects sample volatilization but the column temperature primarily controls the separation. Higher injector temperature (250°C vs 150°C) can lead to faster sample transfer to the column, reducing the time available for differential interactions between enantiomers and the chiral stationary phase. Option A correctly identifies that increased injector temperature reduces interaction time, decreasing chiral selectivity and resolution. Option B incorrectly suggests temperature converts enantiomers to diastereomers, while option C wrongly claims optical rotation determines GC retention. The principle here is that optimal chiral separations require sufficient residence time for the subtle differences in enantiomer-stationary phase interactions to manifest as retention time differences.
11. A racemic compound is separated on a chiral stationary phase via normal-phase HPLC. The analyst replaces the chiral column with an otherwise similar achiral silica column (only variable changed) while keeping mobile phase and flow constant. The chromatogram now shows a single peak. Based on the technique used, which conclusion is most consistent with the results?
    Answer: Enantiomer separation requires a chiral environment; on an achiral stationary phase the enantiomers have identical interactions and co-elute as one peak. This question tests understanding of the fundamental requirement for chiral separation. When the chiral column is replaced with achiral silica, the two enantiomer peaks merge into one because enantiomers have identical interactions with achiral media. The chiral stationary phase provides the three-dimensional environment necessary to differentiate between enantiomers based on their spatial arrangements. Option A correctly explains that enantiomer separation requires a chiral environment, and on achiral phases, enantiomers co-elute due to identical interactions. Option B incorrectly suggests decomposition, while option D wrongly attributes the effect to column efficiency. The fundamental principle is that enantiomers can only be distinguished in chiral environments - without the chiral selector, they behave identically.
12. A racemic compound is subjected to chiral HPLC, but the chromatogram shows two peaks whose integrated areas are 30% and 70%. The sample was prepared by dissolving a weighed solid in solvent; no reaction occurred during preparation. Which statement best describes the outcome of the separation process?
    Answer: The starting material is likely not racemic; the unequal peak areas suggest an enantiomeric excess in the injected sample (assuming equal response factors). This question tests understanding of peak area interpretation in chiral HPLC analysis. The 30:70 peak area ratio indicates the injected sample was not racemic (50:50) but already had an enantiomeric excess. Since no reaction occurred during sample preparation and assuming equal detector response factors for enantiomers, the unequal areas reflect the actual enantiomer composition of the starting material. Option A correctly identifies that the starting material likely had an enantiomeric excess, explaining the 30:70 peak ratio. Option B incorrectly claims enantiomers have different UV absorption, while option C wrongly suggests equal areas are required for enantiomers. The key principle is that chiral HPLC reports the enantiomer ratio present in the sample - it separates but doesn't create or destroy enantiomeric excess.
13. A chiral drug is resolved on a chiral HPLC column. The isolated fraction is reported as 90% ee of the active enantiomer. In a receptor-binding assay, only the active enantiomer contributes to binding; the inactive enantiomer has negligible affinity. Based on the technique used and the meaning of ee, which conclusion is most consistent with the biological implication?
    Answer: Compared with an enantiopure sample of the same total mass, the 90% ee sample will show reduced binding because it contains a substantial fraction of inactive enantiomer. This question tests understanding of enantiomeric excess implications for biological activity when only one enantiomer is active. A sample with 90% ee of the active enantiomer contains 95% active and 5% inactive enantiomer (calculated from ee = (active - inactive)/(active + inactive) × 100%). In receptor binding assays where only the active enantiomer contributes to binding, the presence of 5% inactive enantiomer effectively dilutes the active component, reducing overall binding compared to an enantiopure sample of the same total mass. The correct answer (A) accurately describes this dilution effect where the inactive enantiomer acts as an inert diluent. Choice B incorrectly claims enantiomers have identical interactions with chiral receptors, contradicting the fundamental principle of enantioselective biological recognition. A key insight for pharmaceutical applications is that even high ee values (like 90%) still contain measurable amounts of the unwanted enantiomer, which can impact efficacy or safety profiles.
14. A racemic compound is resolved by preferential crystallization (spontaneous resolution) from an achiral solvent. Seeding with a small crystal of one enantiomer is used to initiate crystallization at constant temperature. Which statement best describes the outcome of the separation process?
    Answer: Seeding can bias crystallization toward one enantiomer in systems that form conglomerate crystals, allowing enrichment without forming diastereomeric derivatives. This question tests understanding of preferential crystallization (spontaneous resolution) for enantiomer separation. This technique works only for racemic compounds that crystallize as conglomerates (separate crystals of each enantiomer) rather than racemic crystals. Seeding with one enantiomer induces crystallization of that enantiomer preferentially, allowing enrichment without forming diastereomeric derivatives. Option A correctly explains that seeding biases crystallization in conglomerate systems, enabling direct enantiomer enrichment. Option B incorrectly claims the method works for any racemate, while option C wrongly requires diastereomer formation. The key principle is that preferential crystallization is limited to the ~10% of racemic compounds that form conglomerate crystals, making it a specialized but derivative-free resolution method.
15. A racemic carboxylic acid is resolved by chiral HPLC. Under condition 1 (mobile phase A: acetonitrile/water 60:40 with 0.1% formic acid), the enantiomers show retention times 6.0 and 6.4 min (poor resolution). Under condition 2, the only change is switching to mobile phase B: acetonitrile/water 40:60 with 0.1% formic acid; retention times become 9.5 and 11.0 min (improved resolution). Based on the technique used, which conclusion is most consistent with the results?
    Answer: Increasing water content strengthens analyte–stationary phase interactions, increasing retention and allowing greater differential binding between enantiomers. This question tests understanding of mobile phase polarity effects on chiral HPLC separation. Increasing water content from 40% to 60% makes the mobile phase more polar, which typically increases retention times on reversed-phase columns but can enhance chiral recognition on many chiral stationary phases. The carboxylic acid analyte shows increased retention (6.0→9.5 min and 6.4→11.0 min) and improved resolution with higher water content. Option A correctly explains that increased water content strengthens analyte-stationary phase interactions, allowing greater differential binding between enantiomers. Option B incorrectly suggests racemization occurs, while option D wrongly claims enantiomers always have identical retention. The key insight is that mobile phase composition significantly affects both retention and chiral selectivity, with more polar conditions often improving resolution for polar analytes on polysaccharide-based chiral phases.
16. A racemic amino acid derivative is separated using a chiral ligand-exchange chromatography method in which Cu(II) forms a complex with the analyte and a chiral ligand bound to the stationary phase. The separation is performed at pH 8.0. When the pH is lowered to 2.0 (only variable changed), the enantiomer peaks collapse into a single peak. Based on the technique used, which conclusion is most consistent with the results?
    Answer: At low pH the analyte is more protonated and less able to coordinate Cu(II), reducing complex formation and eliminating chiral discrimination. This question tests understanding of chiral ligand-exchange chromatography and pH effects on metal coordination. In this technique, Cu(II) forms a ternary complex with both the analyte amino acid and a chiral ligand on the stationary phase, creating diastereomeric complexes that separate. At pH 8.0, the amino acid is largely deprotonated and can coordinate Cu(II) effectively, but at pH 2.0, protonation of the amino group prevents metal coordination. Option A correctly explains that protonation reduces the analyte's ability to coordinate Cu(II), eliminating the chiral discrimination mechanism. Option B incorrectly suggests stronger coordination at low pH, while option C wrongly claims interconversion is required for separation. The principle is that ligand-exchange chromatography requires the analyte to act as a ligand, which depends critically on having available lone pairs for metal coordination.
17. A pharmaceutical intermediate (racemic) is resolved using chiral HPLC with a polysaccharide-based chiral stationary phase. Mobile phase: hexane/isopropanol 90:10 (v/v), flow 1.0 mL/min, UV detection at 254 nm. Two peaks are observed with retention times 4.2 min and 5.6 min. Fractions are collected separately and analyzed by polarimetry in a 1.0 dm cell at 589 nm, 25°C, $c=1.0\ \mathrm{g/100\ mL}$ in ethanol. The early fraction shows $\alpha_{\text{obs}}=+1.20^\circ$ and the late fraction shows $\alpha_{\text{obs}}=-1.18^\circ$. The drug's target enzyme is stereoselective and only one enantiomer is active. Based on the technique used, which conclusion is most consistent with the results?
    Answer: The two peaks correspond to enantiomers that interact differently with the chiral stationary phase, yielding fractions of opposite sign optical rotation and high enantiomeric purity. This question tests understanding of chiral separation using HPLC with a polysaccharide-based chiral stationary phase. In chiral HPLC, enantiomers interact differently with the chiral stationary phase due to their three-dimensional structures, leading to different retention times and allowing separation. The racemic pharmaceutical intermediate shows two peaks at 4.2 and 5.6 minutes, with the collected fractions showing opposite optical rotations (+1.20° and -1.18°), confirming successful enantiomer separation. Option B correctly identifies that the two peaks correspond to enantiomers with opposite optical rotations and high enantiomeric purity. Option A incorrectly suggests diastereomer formation and optical inactivity, while option C makes an unfounded correlation between retention time and biological activity. A key principle in chiral separations is that enantiomers will show equal and opposite optical rotations when pure, and successful chiral HPLC separation yields fractions enriched in each enantiomer.
18. A racemic amine drug candidate is resolved by forming diastereomeric salts with enantiopure (R)-tartaric acid, followed by crystallization from ethanol at 20C. The less soluble salt crystallizes first and is isolated, then basified to regenerate the free amine. The isolated free base shows $\alpha_{obs}=-6.0^\circ$ in a 1.00 dm cell at $c=2.0\ \text{g}/100\ \text{mL}$ (same solvent and wavelength as the literature value). The pure (R)-amine has $[\alpha]= -6.0$ under these conditions. Which statement best describes the outcome of the separation process?
    Answer: The crystallized material is optically pure (R)-amine after regeneration from the salt. The skill being tested is enantiomer resolution via diastereomeric salt formation and fractional crystallization. This technique involves reacting a racemic mixture with an enantiopure resolving agent to form diastereomers with differing solubilities, allowing selective crystallization. Here, the racemic amine forms salts with (R)-tartaric acid, and the less soluble salt is isolated and basified to yield the free amine with rotation matching the pure (R)-enantiomer. Choice B accurately reflects that the crystallized material is optically pure (R)-amine post-regeneration, as the matching rotation confirms complete enrichment. A common distractor like choice A fails by assuming crystallization cannot alter enantiomeric composition, ignoring that diastereomeric salts enable separation due to physical property differences. A transferable insight is to compare observed rotation to literature values under identical conditions to assess purity after regeneration. In similar resolutions, selecting an appropriate chiral acid and solvent optimizes yield and purity of the desired enantiomer.
19. A racemic chiral drug is resolved by chiral HPLC for clinical development. Regulatory specifications require $\ge 98\%$ ee of the active enantiomer. A batch shows $\alpha_{obs}=+19.6^\circ$ (1.00 dm, $c=1.0\ \text{g}/100\ \text{mL}$); the pure active enantiomer has $\alpha_{obs}=+20.0^\circ$ under identical conditions. Based on the technique used, which conclusion is most consistent with the results?
    Answer: The batch is ~98% ee and meets the specification, assuming no other optically active impurities contribute. The skill being tested is determining enantiomeric excess from polarimetry data post-chiral HPLC resolution. Ee is calculated from the ratio of observed to pure rotations under identical conditions, assuming no impurities. Here, the batch's +19.6° versus +20.0° for pure indicates ~98% ee, meeting specifications. Choice A correctly concludes the batch is ~98% ee and compliant. A common distractor like choice B fails by deeming it racemic, ignoring the near-match to pure. A transferable insight is to account for measurement error in ee calculations. In similar quality controls, combining polarimetry with chiral HPLC ensures accurate purity assessment.
20. A chemist attempts to separate the enantiomers of a chiral carboxylic acid (Compound D) using standard reversed-phase HPLC with an achiral C18 column (no chiral additives). The mobile phase is 60:40 water:acetonitrile with 0.1% formic acid, and UV detection is used. A single symmetric peak is observed for the injected racemate. The chemist then repeats the run on the same achiral column but with a different injection volume; still only one peak is observed. Which statement best describes the outcome of the separation process?
    Answer: The enantiomers co-elute because an achiral stationary phase provides identical interactions for both enantiomers under these conditions. This question tests understanding of the fundamental limitation of achiral chromatography for enantiomer separation. On a standard reversed-phase C18 column without chiral additives, enantiomers experience identical interactions with the achiral stationary phase because they have identical physical properties in achiral environments. This results in identical retention times and complete co-elution as a single peak. The carboxylic acid enantiomers have the same hydrophobicity, polarity, and molecular shape in the achiral column environment. Choice C incorrectly suggests UV detection is the limiting factor, but the issue is chromatographic separation, not detection. Choice B wrongly proposes meso compound formation, which would require breaking and forming covalent bonds. The key principle is that enantiomer separation requires a chiral environment - either a chiral stationary phase, chiral mobile phase additive, or derivatization with a chiral reagent to form separable diastereomers.

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